hey there.

no this isn't that simple I regret...the 6800GT is "proportionally" better than the ultra. because : it needs less power.
It has the same nmber of pixel pipeline (16) but has a little slower fillrate, and functions @350/1000mhz where the 6800Ultra is @420/1150.
When overclocking the GT to ultra's latencies, you get higher fps in the games,, but low stability if you don't have a correct power supply and/or thermic paste.
considering the ultra is 100 bucks more than the gt , i recommend you take the gt and an overclokcing warranty (just if it burns

) it costs you less than the ultra alone.
I hope i did help you a bit.
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I don't think that:
"Time is a predator, that seeks us all our life..."
But that :
"time is a companion, who takes part of our great trip through life..."
Huhuh, Star Trek Fan